For the apples: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch glass baking dish with butter.
Arrange the apple slices in the prepared baking dish in an even layer. In a bowl, mix together the granulated sugar, flour, cinnamon and salt. Sprinkle the mixture over the apples, then pour 1/2 cup water over top.
For the topping: In a separate bowl, mix the flour, oats, brown sugar, baking powder, baking soda and a pinch of salt. Stir while drizzling in the melted butter. Sprinkle the topping in an even layer over the apples.
Bake until the topping is slightly crisp and golden brown, about 43 minutes. Serve warm or at room temperature on its own, with ice cream or with frozen custard.